Most pthread functions return 0 on success and non-zero on error, but those
errors might be positive or negative, so checking for return values in the
Unix style, where errors are less than zero, is a bug.

This patch allows the user to disable the behaviour that blocks the compositor through a new hint: SDL_VIDEO_X11_NET_WM_BYPASS_COMPOSITOR. This allows tools or other windowed applications to behave properly under KWin.